The invention relates to zero-degree posterior teeth for a lower and an upper denture, wherein opposite posterior teeth interact at least substantially in line contact.
A patient who is provided with complete dentures proves to be unable to increase the forces which he used to exert with his natural dentition for reducing the size of the food particles and for preparing the food for deglutition. Whenever the dentures cause comfortable sensations, such as pain, as frequently occurs with ill-fitting dentures, the forces for mastication will even be reduced reflectorily. With well fitting dentures the total amount of the exerted forces for mastication will generally lie within narrow limits.
Further, the tissuebed on which the dentures are seated is not intended by nature to be used as support face and thus should be relieved as much as possible, whilst nevertheless all forces which are exerted on the posterior teeth are fully transferred to this tissuebed. It is evidently of the greatest importance that the total amount of forces needed for the reduction of the size of the food particles, is reduced to a minimum, or, in other words, that the exerted forces are utilized to the utmost.
In a known embodiment of zero-degree posterior teeth for a lower and an upper denture the active surface (or end surface) of the lower posterior teeth is formed as a central mesio-distally directed, zigzag or straight, mastication ridge, which interacts with a flat active surface of the opposite upper posterior teeth.
With these known posterior teeth each pair of opposite teeth is in line contact with each other. This is based on the consideration that by reducing the contact area between the active surfaces of opposite posterior teeth, the specific surface pressure increases, which promotes the effect of mastication, i.e. the cutting of the food.
The design of the known posterior teeth as zero-degree posterior teeth--which expression means to indicate posterior teeth, each having the elements performing the cutting action lying in one flat surface (the active surface) of the posterior tooth concerned--has the advantage that the normal functional jaw movement during mastication is not obstructed in any way due to the fact that the transversal mobility is optimal. Further, the set-up of zero-degree posterior teeth in lower and upper dentures is considerably easier than the set-up of posterior cusp teeth, as the active surfaces of the zero-degree posterior teeth in the lower or upper denture are situated in one and the same flat surface.
Although the forces exerted by these known zero-degree posterior teeth for lower and upper dentures are generally efficiently utilized for the cutting of the food, these posterior teeth nevertheless show certain disadvantages.
With the known posterior teeth for lower and upper dentures the cutting action is so predominant that the contour of the active surface (the masticatory surface) of the natural posterior teeth has been abandoned completely and has been sacrificed to the cutting capacity. The extent of the contour of the known posterior teeth is significantly smaller than that of the contour of the natural posterior teeth, to such a degree that hardly a contour is left.
However, in this connection it should be considered that the mastication of food with the natural posterior teeth does not solely consist of a cutting action of the food, but should be, so to speak, a combination of cutting, pressing and mixing actions.
It is of great importance that the consumed food has a pleasant taste, not only for this fact alone, but also because in this way the stimulus for the reduction of the size of the food particles may be reflectorily raised optimally.
The pressing action intends to cause an intensive mixing of the saliva with the food and further serves to rapidly release the juices and flavors from the cells in the food, which cells are opened by the cutting action.
With the known zero-degree posterior teeth as described hereinbefore the mandatory pressing and mixing actions for preparing the food to a deglutable pulp are only obtained to a minor extent. These posterior teeth have an insufficient grip of the food because of a serious lack of active surface area. This implies that an increased tongue muscle action is required to compensate for the decreased grip of the posterior teeth on the food.